The present invention relates to gas lift completion systems for subsea oil wells and, more specifically, to a concentric tubing completion system in which the annulus around the production tubing string within the production riser is used to supply lift gas to the well.
In many subsea oil wells, the pressure within the well is not sufficient to force the oil out of the formation and up the production riser to the Christmas tree which is mounted on a structure such as a vessel or a platform that is located at the surface of the ocean. In such situations, a gas lift method is commonly employed to help the oil reach the tree. In order to utilize this method, high pressure gas must be conveyed from the surface structure to the annulus between the production tubing and the well casing. Currently, the gas for gas lift applications is typically supplied to the subsea well by a second tubing string which extends parallel to the production tubing string within the production riser. However, this type of system requires the use of a dual bore Christmas tree, a dual string tubing hanger and special tubing handling equipment, which results in increased complexity and cost for the completion system.
The present invention addresses these limitations in the prior art by providing a concentric tubing completion system for communicating a fluid between a surface structure and a mudline wellhead which is installed at the upper end of a subsea well bore. The completion system comprises a surface christmas tree which is located on the surface structure, a tieback connector which is connected to the mudline wellhead, and a mudline tubing hanger which is supported in the tieback connector and from which is suspended a tubing string that extends into the well bore and defines a tubing annulus surrounding the tubing string. The concentric tubing completion system also comprises an outer tubing hanger which is supported by the surface Christmas tree, an inner tubing hanger which is supported by the surface Christmas tree, an outer tubing string which is connected between the outer tubing hanger and the mudline tubing hanger, and an inner tubing string which is disposed within the outer tubing string and which is connected between the inner tubing hanger and the mudline tubing hanger.
In accordance with the present invention, the inner and outer tubing strings define an annulus between them which communicates with a number of annulus passageways that extend through the mudline tubing hanger to the tubing annulus. Therefore, the fluid may be communicated between the surface christmas tree and the tubing annulus through the annulus between the inner and outer tubing strings. In gas lift applications, this eliminates the need for an independent, parallel tubing string within the production riser for communicating gas from the surface christmas tree to the tubing annulus. Consequently, a dual bore christmas tree, a dual string tubing hanger and special tubing handling equipment are not required with the concentric tubing completion system of the present invention.
The concentric tubing completion system may also include one or more devices for adjusting the tension in the inner and outer tubing strings. For example, the outer tubing string may be connected to the outer tubing hanger with a ratch-latch mechanism. Similarly, the inner tubing string may be connected to the inner tubing hanger with a ratch-latch mechanism. In this manner, the tension in both the inner and outer tubing strings may be independently adjusted at the surface structure.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are sometines used to denote similar components in the various embodiments.